| 摘要 |
<p>A method for treating the pipe end of a metal pipe material in which a threaded portion can be formed with high precision dimensions while preventing occurrence of flaw on the inside at the pipe end when the threaded portion is formed at the pipe end of the metal pipe material. A plug for use therein is also provided. Before a threaded portion is formed at the pipe end of a metal pipe material (2), a process for enlarging a pipe is carried out using a plug (1) having such a sliding portion (1a) as the diameter of cross-section decreases from a position X(D) in the central axis direction where the diameter of cross-section becomes maximum diameter D toward the distal end and the taper angle ? of a taper surface S, i.e. the outer surface of the sliding portion (1a), for the central axis M is not larger than 3.5° (i.e. ?=3.5°) between a position X(0.7D) in the central axis direction where the diameter of cross-section becomes 0.7D and the position X(D) in the central axis direction. In this process for enlarging a pipe, the contact angle a(equal to the taper angle ?) becomes 3.5° or less when the taper surface S of the plug (1) touches the pipe end of the metal pipe material (2).</p> |
